National Geographic : 1985 Nov
Kenya called Ramapithecusand Sivapithe cus; and two from Europe called Ruda pithecus and Dryopithecus. These apelike creatures lived at various times between about 8 and 20 million years ago. Despite much debate and speculation, none of these primates has been finally ac cepted as a human progenitor. Until more fossils-and more complete specimens-are found, the long geologic epoch known as the Miocene (24 million to 5 million years ago) will remain a largely veiled chapter in homi nid evolution. Certain events near the end of the Mio cene and the beginning of the next epoch, the Pliocene, may help answer the second ques tion: What was the driving force that made these shadowy figures of the remote past give up an arboreal life and become upright, two-legged terrestrial hominids? C. K. (Bob) Brain, director of the Trans vaal Museum in Pretoria, South Africa, be lieves that radically changing environment forced these forest primates to adapt. Brain developed his hypothesis through a study of what is called the Terminal Miocene Event. He finds a remarkable record of cat astrophic environmental change, a pro found cooling between five and six million years ago, following millions of years of mostly warm climate. This sharp climatic change is documented by analysis of deep sea cores and terrestrial deposits, by fossil pollen and land snails, and by dramatic changes in mammalian life. The drastic plunge in temperatures pro duced a rapid buildup of ice in Antarctica. The enormous ice sheets took up so much water that sea levels worldwide dropped 50 to 60 meters (165 to 195 feet). Rainfall in many places was strongly affected. Across wide regions of tropical Africa, warm turned cool and wet became arid. The heavily wooded areas of the past retreated steadily, giving way to advancing grass lands and scattered clumps of low trees much like the savannas of East Africa to day. The habitats of ancestral apes shrank alarmingly. At about the same time, according to fos sil records gathered by Elisabeth S. Vrba at the Transvaal Museum, evolutionary changes seem to have taken place in African fauna. Unable to cope with the hostile open 582 A fossil's long journey HANCES FOR SURVIVAL of a hominid bone, perhapsone in a billion, depend on particularsets of geologic circumstances,one of which is demonstrated by these drawings.An early man collapses (above) beside a riverfringed by lush galleryforest. Hyenas and otherscavengers close in andpull the carcassapart.The large, hard skull, in this instance, tumbles downstream in aflash flood and is d"i. ~ r,P.